Hacker News Reader

Comment by rijoja | Hacker News Reader

If one wanted to go about and take the easy way out and instead of using discrete chips going with an FPGA. Which FPGA-chip / board would be a suitable option?

View on HN

At the very top end, the VC707 ($3500) will let you run a quad core 64 bit RISC-V at about 100 MHz. At the bottom end you'd be looking at one of the boards supported by PicoRV32 (https://github.com/cliffordwolf/picorv32), and some of those are as little as $100. If you want flashing lights as in the video then the Nexys 4 DDR might be a good compromise (but you'd have to program the lights to flash yourself).

100 MHz, why so slow?

The very quick explanation is that when you have lots of long wires (and especially wires of different lengths) between the components of your chip, you have to wait for all the signals on a bus to arrive before you can latch them into the component at the next clock edge, and this slows down the achievable clock speeds.

To explain this properly would require explaining how FPGAs work and how they differ from ASICs. There's a rather good talk by Uli Drepper which covers some FPGA basics: https://www.youtube.com/watch?v=Q-SkioVSZWw

In any case 100 MHz for an FPGA processor design is not slow for an FPGA, nor is it something that can be solved with more money (bigger and smaller FPGAs suffer the same effect). For GHz designs you have to implement an ASIC. For comparison, SiFive's FPGA implementation runs at 100 MHz but their ASIC of near-exactly the same design runs at 1.5 GHz.

I've used this little guy to run experiments with the PicoRV32:

https://www.aliexpress.com/item/New-Mini-System-Development-...

The board is less than $15. Add a JTAG cable clone for another $10, and you're all set!

I would say Xilinx artix-7 board is good enough to experiment with picorv32 and it costs only $100.

OTOH, if you go for a lattice ICE40 based board, you can use a fully open source toolchain.

I have a icoboard connected to a raspberry Pi 3 -- it's my little self-contained FOSS FPGA development station. Quite convenient :)

> OTOH, if you go for a lattice ICE40 based board, you can use a fully open source toolchain.

But you won't be able to synthesize a RISC-V CPU. The iCE40 series consists of relatively small parts.

> But you won't be able to synthesize a RISC-V CPU. The iCE40 series consists of relatively small parts.

You can fit picorv32 on an iCE40-HX8K, although admittedly you'll only get an RV32IMC core with just the user ISA.

Reply on news.ycombinator.com